The present invention relates to a return speed control system for use in a document scanning apparatus. More particularly, this invention relates to a return speed control system with a moving means for optically scanning a document placed on a document table of an image forming apparatus such as a copy machine.
In a copy machine, for example, the document scanning apparatus is generally classified into two types, a document table moving type and a document table fixed type. The latter type of document scanning apparatus is usually constructed such that a moving means containing an exposure lamp and a mirror moves in parallel with and along the document table. In this apparatus, the copying speed is one of the factors which can be improved. The best way to improve the copying speed is to increase the return speed of the moving means to be as fast as possible, because the return time taken for the return of the moving means never contributes to the copying operation itself.
Simply increasing the return speed, however, is not sufficient. For example, it hinders the smooth return of the moving means to its initializing or home position. If the moving means is abruptly and rapidly moved as it returns, after a short time, the drive force applied to the moving means is lost. If this happens, the inertia and resilient force still remain in the moving means after the drive force is lost, which causes the moving means to overrun the home position and to violently hit the member located near the home position. The great shock caused by the moving means hitting the member can possibly damage or destroy the exposure lamp and mirror of the moving means, and can distort or loosen the fixing portions of those components.
The damage or distortion of the moving means results in poor copy, e.g., deformation of the copied image, which makes it impossible to scan the document and thus to effect the copying operation.
The above problem is involved not only in the return operation, but also in the reset operation performed when the power is on. In the document table-fixed type of document scanning apparatus, the moving means must be at the home position at the start of copying. Accordingly, when the copy machine is in a copy ready mode before the copy operation starts, the moving means is usually at the home position. When the copy machine is in such a mode, the moving means is frequently displaced from the home position. Therefore, it is necessary to return the moving means to the home position before the copy operation begins.
Recently many copy machines have employed a frame halving structure for ease of maintenance. In this structure, the main frame is separable into two individual sections with respect to the paper transfer path of the machine. One of the sections may be hingedly turned for opening when trouble such as a paper jam occurs, so as to allow an operator to easily have access to the trouble spot in the machine to remove the paper jam. In this type of copy machine, when one of the sections is hingedly turned for opening, the moving means located at the distal end of the section moves down, due to its weight, from the home position in the document scanning direction. Accordingly, the moving means has an indefinite initial position when the power is on. It is for the above reasons that the reset operation is required for the moving means when the power is on.
Usually, the reset operation, like the return operation as mentioned above, is performed at a relatively high speed. Therefore, this reset operation is accompanied by many problems similar to the above.
To solve those problems, various types of return speed control systems have been proposed. In a conventional return speed control system, the moving means is first moved at a high speed and is stopped at a point near the home position, and then is moved again at a low speed up to the home position. From the viewpoint of improving the copying speed, however, this control system is still defective in that it stops the moving means during its movement which afterwards must be slowed down.